Use GitHub-Flavored Markdown alert syntax for Note/Tip/Warning/Important blocks

Markdown export now emits > [!NOTE]/[!TIP]/[!WARNING]/[!IMPORTANT] for
text-block callouts, giving them the same colored/icon-labeled treatment
as the PDF/HTML/DOCX exports on renderers that support GFM alerts
(GitHub, Azure DevOps wikis, etc.), while degrading gracefully to a
plain blockquote elsewhere.

tests/unit/exporters-text.test.js

@@ -109,10 +109,11 @@ test('Markdown export: TOC anchors resolve, images exist, blocks rendered', (t)
   assert.equal(lines[fenceStart + 1], '0 2 * * * /usr/local/bin/acmesync --backup');
   assert.equal(lines[fenceStart + 2], '```');
   assert.ok(lines.some((l) => /^\| Day \| Window \|$/.test(l)), 'table header row');
-  // Warning text block became a blockquote with its content.
-  const warnIdx = lines.findIndex((l) => l.startsWith('> **Warning: Access**'));
+  // Warning text block became a GFM alert blockquote with its content.
+  const warnIdx = lines.findIndex((l) => l.startsWith('> [!WARNING]'));
   assert.ok(warnIdx > 0);
-  assert.equal(lines[warnIdx + 1], '> Admins only.');
+  assert.equal(lines[warnIdx + 1], '> **Access**');
+  assert.equal(lines[warnIdx + 2], '> Admins only.');
 });
 
 test('Confluence export writes storage-format XML and image attachments', (t) => {